The network of al-Qa'ida sympathisers stretching from Leicester to Afghanistan was carefully and slowly put together. It collapsed because of one incident at an immigration desk in the United Arab Emirates.

On 28 July last year, immigration officers in Dubai detained Djamel Beghal, a French-Algerian travelling as a businessman, for suspected passport irregularities. They began questioning him about his movements. And, it appears, he quickly began to talk.

They discovered that Mr Beghal, 35, was a key associate of Osama bin Laden, and had intimate knowledge of an intricate web of terrorist activists and their supporters which stretched across Europe and ended last week in the Highfields district of Leicester.

Mr Beghal was quickly handed over to the French counter-intelligence service, the DST, who insist he continued to confess of his own free will. Lawyers for Mr Beghal allege their client was tortured before making these confessions, but the French say that after Muslim clerics in Dubai persuaded him that terrorism was un-Islamic, he turned super-grass.

Mr Beghal admitted his stop in Dubai was en route from a meeting with Mr bin Laden's closest associates in Afghanistan. He also confessed to planning a suicide bombing at the US embassy in Paris in March this year.

Crucially, Mr Beghal also named at least seven key associates, including Kamel Daoudi, 23, another French Algerian. These core suspects are alleged members of the Algerian radical Islamic group Takfir-wal-Hijra.

It took the devastating bombings of 11 September to spur action. The French raided Mr Daoudi's Paris flat on 20 September but he had already fled to Leicester, where it later emerged that his al-Qa'ida cell had its main British base. Tipped off by the DST, armed police and MI5 officers arrested Mr Daoudi in Leicester on 26 September. Seven other arrests were made in France, Belgium and Holland.

The intelligence services claim Mr Beghal helped recruit three alleged suicide bombers – Richard Reid, the British "shoe bomber" facing life sentences in the US, Zacharias Moussaoui, who was supposedly due to be the 20th hijacker on 11 September, and a Tunisian-born former footballer, Nizar Trabelsi. All three were allegedly spotted with Mr Beghal at mosques in London run by the radical imams Abu Hamza al Masri and Abu Qatada.

In Highfields, Mr Beghal apparently cultivated young Muslims through a local mosque. He also quietly built up the network of supporters among foreign émigrés, who used the local Indian and Pakistani communities as cover. These allegedly include the nine men, aged 23 to 40, who are being held on terrorism charges after raids in the east Midlands city and in London on Thursday and Friday last week. Another eight people were charged with immigration offences alone.

The Security Service does not think, however, those arrested were all members of an al-Qa'ida cell. One Whitehall source explained: "MI5 believes that these people were all supporters of groups related to al-Qa'ida but not active terrorists. This was a fundraising network which got its money from credit card frauds."

Their arrests were foreshadowed by the appearance in court on Thursday of two of Mr Daoudi's alleged associates in Leicester, originally arrested for immigration offences on the same day. Baghdad Meziane, 36, and Brahim Benmerzouga, 30, both Algerians, were remanded in custody, charged with membership of al-Qa'ida and inciting terrorism overseas.

Since 11 September, roughly 130 people have been arrested under the Terrorism Act 2000, as well as the eight interned on 19 December under the Home Secretary's new powers of detention without charge. Some have been Irish republicans and others have links with to the Palestinian group Hamas. There are major doubts about the justification for some of the arrests. One man arrested under the new internment powers last month, Djamel Ajoaou, was allowed to fly home to Morocco after it emerged in court that MI5 had failed to discover he was a Home Office-cleared prison translator. Many, however, appear to have links with the events of 11 September.

Two men appeared in court in Birmingham on Friday, charged with possession of explosives and plotting to cause explosions in Britain. Dr Faisal Mostafa, 37, is accused of having a "Mujahedin Explosives Handbook" and another bomb-making guide on his computer. Moinul Abedin, 31, is accused of possessing ingredients for home-made bombs at two local addresses.

One of the difficulties facing security services is that though some suspects belong to al-Qa'ida, there are many more who belong to groups associated with al-Qa'ida or who support Mr bin Laden but act alone.

"It is the last group that is most difficult to spot, the greatest headache and potentially the nightmare for intelligence officers," said one intelligence source.